1. Field of the Invention
The invention relates to a method for forming a nitride or oxide film electrode on a solid electrolyte, and more specifically to a method for forming a nitride or oxide film electrode on beta"-alumina for use in a thermo-electric generator.
2. Description of the Related Art
Thermo-electric generators convert heat energy from a heat source to electricity. Examples of thermo-electric generators are the sodium heat engine (SHE), alkali metal thermo-electric converter (AMTEC), or liquid metal thermo electric converter (LMTEC), which generate electricity by expanding a metal, for example sodium, across a solid electrolyte. In such generators, the solid electrolyte separates a closed container into a first and second reaction zone. The first reaction zone contains liquid sodium, while the second reaction zone contains an electrode in contact with the solid electrolyte. As the first reaction zone is heated, the liquid sodium gives up electrons, causing sodium ions to migrate through the electrolyte to the second reaction zone, to be neutralized at the electrode-electrolyte interface. Neutralized sodium metal atoms pass through the electrode, then evaporate, pass through a condenser, and are returned to the first reaction zone.
The efficiency of the electrode in the thermo-electric generator described above has been optimized by use of permeable materials which do not impede migration of the sodium atoms through the electrode, while simultaneously conducting electrons to the solid electrolyte interface to neutralize the sodium ions migrating through the electrolyte. A good example of a suitable electrode displaying the above features is a porous metal film, such as a nitride, deposited on a ceramic electrolyte.
Various methods have been proposed for producing an electrode such as the one described above. Such methods include reactive sputtering, ion plating, electron beam evaporation, and the like. Such methods, however, have a number of drawbacks when used to produce a metal film electrode on a ceramic substrate for use in a thermo-electric generator.
For example, the conventional methods frequently degrade the mechanical or ion-conducting properties of the ceramic substrate.
Further, the conventional methods produce a film which does not adhere well to the ceramic substrate over the life of the electrode.
Further, the conventional methods are relatively expensive and extremely time consuming, producing a serious commercial disadvantage.
Further, the conventional methods produce an extremely dense film on the ceramic substrate. It would be advantageous for the metal film to be as thick as possible, since such an electrode would have a low sheet resistance to electron flow. However, due to the high density of the film produced by the conventional methods, increasing thickness results in high impedance to diffusion of neutralized sodium atoms. Thus, using the conventional methods, only a thin electrode can be produced which has any reasonable commercial use.
The present invention improves on the conventional methods by providing a method for bonding a film electrode on a solid electrolyte that does not degrade the mechanical or ion-conducting properties of the electrolyte, adheres well to the electrolyte, is quick and inexpensive, and produces thicker films, thus improving conductivity without impeding the diffusion of sodium atoms.
Additional advantages of the invention will be set forth in the description which follows and in part will be obvious from the description, or may be learned from practice of the invention. The advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.